THE ACCUMULATOR
If we had two large water tanks, one of which could be emptied only
by allowing the bottom to fall completely out, and the other by means
of a narrow pipe, it is easy to see which would be the more useful to
us as a source of water supply. If both tanks were filled, then from
the first we could get only a sudden uncontrollable rush of water,
but from the other we could get a steady stream extending over a long
period, and easily controlled. The Leyden jar stores electricity,
but in yielding up its store it acts like the first tank, giving a
sudden discharge in the form of a bright spark. We cannot control the
discharge, and therefore we cannot make it do useful work for us. For
practical purposes we require a storing arrangement that will act like
the second tank, giving us a steady current of electricity for a long
period, and this we have in the accumulator or storage cell.
A current of electricity has the power of decomposing certain liquids.
If we pass a current through water, the water is split up into its
two constituent gases, hydrogen and oxygen, and this may be shown by
the apparatus seen in Fig. 12. It consists of a glass vessel with two
strips of platinum to which the current is led. The vessel contains
water to which has been added a little sulphuric acid to increase its
conducting power, and over the strips are inverted two test-tubes
filled with the acidulated water. The platinum strips, which are
called _electrodes_, are connected to a battery of Daniell cells.
When the current passes, the water is decomposed, and oxygen collects
at the electrode connected to the positive terminal of the battery,
and hydrogen at the other electrode. The two gases rise up into the
test-tubes and displace the water in them, and the whole process is
called the electrolysis of water. If now we disconnect the battery and
join the two electrodes by a wire, we find that a current flows from
the apparatus as from a voltaic cell, but in the opposite direction
from the original battery current.
[Illustration: FIG. 12.--Diagram showing Electrolysis of Water.]
It will be remembered that one of the troubles with a simple voltaic
cell was polarization, caused by the accumulation of hydrogen; and that
this weakened the current by setting up an opposing electro-motive
force tending to produce another current in the opposite direction.
In the present case a similar opposing or back electro-motive force
is produced, and as soon as the battery current is stopped and the
electrodes are connected, we get a current in the reverse direction,
and this current continues to flow until the two gases have recombined,
and the electrodes have regained their original condition. Consequently
we can see that in order to electrolyze water, our battery must have an
electro-motive force greater than that set up in opposition to it, and
at least two Daniell cells are required.
This apparatus thus may be made to serve to some extent as an
accumulator or storage cell, and it also serves to show that an
accumulator does not store up or accumulate electricity. In a voltaic
cell we have chemical energy converted into electrical energy, and here
we have first electrical energy converted into chemical energy, and
then the chemical energy converted back again into electrical energy.
This is a rough-and-ready way of putting the matter, but it is good
enough for practical purposes, and at any rate it makes it quite clear
that what an accumulator really stores up is not electricity, but
energy, which is given out in the form of electricity.
The apparatus just described is of little use as a source of current,
and the first really practical accumulator was made in 1878 by Gaston
Planté. The electrodes were two strips of sheet lead placed one upon
the other, but separated by some insulating material, and made into a
roll. This roll was placed in dilute sulphuric acid, and one strip or
plate connected to the positive, and the other to the negative terminal
of the source of current. The current was passed for a certain length
of time, and then the accumulator partly discharged; after which
current was passed again, but in the reverse direction, followed by
another period of discharge. This process, which is called _forming_,
was continued for several days, and its effect was to change one plate
into a spongy condition, and to form a coating of peroxide of lead on
the other. When the plates were properly formed the accumulator was
ready to be fully charged and put into use. The effect of charging was
to rob one plate of its oxygen, and to transfer this oxygen to the
other plate, which thus received an overcharge of the gas. During the
discharge of the accumulator the excess of oxygen went back to the
place from which it had been taken, and the current continued until the
surfaces of both plates were reduced to a chemically inactive state.
The accumulator could be charged and discharged over and over again as
long as the plates remained in good order.
In 1881, Faure hit upon the idea of coating the plates with a paste of
red-lead, and this greatly shortened the time of forming. At first it
was found difficult to make the paste stick to the plates, but this
trouble was got rid of by making the plates in the form of grids, and
pressing the paste into the perforations. Many further improvements
have been made from time to time, but instead of tracing these we will
go on at once to the description of a present-day accumulator. There
are now many excellent accumulators made, but we have not space to
consider more than one, and we will select that known as the “Chloride”
accumulator.
The positive plate of this accumulator is of the Planté type, but it is
not simply a casting of pure lead, but is made by a building-up process
which allows of the use of a lead-antimony mixture for the grids. This
gives greater strength, and the grids themselves are unaffected by the
chemical changes which take place during the charging and discharging
of the cell. The active material, that is the material which undergoes
chemical change, is pure lead tape coiled up into rosettes, which are
so designed that the acid can circulate through the plates. These
rosettes are driven into the perforations of the grid by a hydraulic
press, and during the process of forming they expand and thus become
very firmly fixed. The negative plate has a frame made in two parts,
which are riveted together after the insertion of the active material,
which is thus contained in a number of small cages. The plate is
covered outside with a finely perforated sheet of lead, which prevents
the active material from falling out. It is of the utmost importance
that the positive and negative plates should be kept apart when in the
cell, and in the Chloride accumulator this is ensured by the use of a
patent separator made of a thin sheet of wood the size of the plates.
Before being used the wood undergoes a special treatment to remove
all substances which might be harmful, and it then remains unchanged
either in appearance or composition. Other insulating substances, such
as glass rods or ebonite forks, can be used as separators, but it is
claimed that the wood separator is not only more satisfactory, but that
in some unexplained way it actually helps to keep up the capacity of
the cell. The plates are placed in glass, or lead-lined wood or metal
boxes, and are suspended from above the dilute sulphuric acid with
which the cells are filled. A space is left below the plates for the
sediment which accumulates during the working of the cell.
In all but the smallest cells several pairs of plates are used, all the
positive plates being connected together and all the negative plates.
This gives the same effect as two very large plates, on the principle
of connecting in parallel, spoken of in Chapter IV. A single cell, of
whatever size, gives current at about two volts, and to get higher
voltages many cells are connected in series, as with primary cells.
The capacity is generally measured in ampere-hours. For instance, an
accumulator that will give a current of eight amperes for one hour, or
of four amperes for two hours, or one ampere for eight hours, is said
to have a capacity of eight ampere-hours.
Accumulators are usually charged from a dynamo or from the public
mains, and the electro-motive force of the charging current must be
not less than 2½ volts for each cell, in order to overcome the back
electro-motive force of the cells themselves. It is possible to charge
accumulators from primary cells, but except on a very small scale the
process is comparatively expensive. Non-polarizing cells, such as the
Daniell, must be used for this purpose.
The practical applications of accumulators are almost innumerable,
and year by year they increase. As the most important of these are
connected with the use of electricity for power and light, it will be
more convenient to speak of them in the chapters dealing with this
subject. Minor uses of accumulators will be referred to briefly from
time to time in other chapters.